The purpose of this study is to determine why a definite pattern of serum corticosterone secretion becomes evident in young rats at about weanling age. In adults, several neural inputs to the hypothalamus, including the retinohypothalamic path, the fornix and the medial forebrain bundle have been found to influence the adrenal rhythm. This proposal is directed toward the general question of how so many inputs normally interact to control an endocrine rhythm. These goals will be achieved by comparing the development of other rhythmicities (sleep-wakefulness, body temperature and food intake) with the patterns worked out for the adrenal rhythm and to determine whether these rhythms are more susceptible to modulation in particular stages of development. Trace the development of median raphe connections and fornix connections to the hypothalamus to determine the time course of development in relationship to the adrenal and body temperature rhythms. Lesion of the suprachiasmatic nuclei will be performed at different stages and determined if the biological rhythms appear or reappear with a delayed time course. Bibliographic references: Ramaley, J.A. and D. Bartosik. Precocious puberty: The effect of adrenalectomy on PMS-induced ovulation and progesterone secretion. Endocrin. 96:269-274, 1975. Ramaley, J.A. Adrenal rhythmicity in the immature pseudopregnant rat. Proc. Soc. Exp. Biol. Med. 148:762-764, 1975.